The PO changed the zinc every year and still look at the build up of zinc in the exchanger.
By your picture, it looks to me like the build up of zinc (actually the pile of it) is because the PO did not inspect/change the zincs frequently enough. In a addition to getting generally smaller in size while doing its job, a new pencil zinc becomes spongy and soft as it sacrifices. It can then separate from the brass fitting if it is not changed out soon enough. Once the zinc separates from it's brass holder, it no longer has an firm electrical connection to the heat exchanger metal. So it no longer sacrifices and doesn't waste away very well any more. Just sits there to eventually block the tubes. So perhaps when the PO did the zinc annual replacement, either the previous year's pencil zinc had already broken off from the brass holder or would do so as the PO was removing it. The PO then would just insert a new pencil zinc without removing the left inside residue.
I've read somewhere that heat exchanger zincs should be almost a monthly inspect/change item. I doubt that very many of us, me included, do it that frequently. I know that after only two/three months in my heat exchanger, the zinc is just a shadow of its former self.
Any acid wash I don't think is to clear out pencil debris, but to dissolve general scale build up. I would do a lot more research before using anything stronger than vinegar. If done wrong, acid can attack weak points in the heat exchanger and probably will decrease its total useful life. I've been told that radiator shops will use another chemical right after the acid to pacify the reaction quickly.
I had a bad "acid" experience right after I bought my boat. Probably the exchanger on my boat was already leaking between the coolant and seawater circuits. But I certainly made it worse when I gave it a weak muriatic acid bath to make it clean for the proud new owner. The exchanger's weak area was the "baffle" plate at each end into which the tubes are brazed. The tubes themselves still were in great shape. Fortunately a local radiator shop with marine heat exchanger experience was able to flood the weak areas with solder to seal the breaches. The exchanger has been working fine now for quite a few years.
). The PO changed the zinc every year and still look at the build up of zinc in the exchanger. 
